1. Field of the Invention
The present invention generally concerns a process of granulation and an apparatus therefor, and more particularly it concerns an improved process of granulation for obtaining comparatively large granules and apparatus therefor.
2. Description of the Prior Art
There are known in the art apparatus with a large capacity for mass-producing small sized granules and capable of manufacturing high quality products with a single unit. For instance, in the prilling system used in granulation of urea, etc., one unit has a capacity as large as 2,000 t/day or more. However, it is impossible with such an apparatus to increase the granule diameter of the product beyond a certain limit because of the physical properties of the melt to be granulated.
The reason for this is because, if the diameter of the nozzle for the melt is increased, the melt flows continuously and the liquid drops cannot be formed. If the liquid drops become larger, on the other hand, the vertically falling distance of the liquid drops required for solidification becomes excessive. According to the prilling process, the average diameter of ca. 2 mm is the limit for granular urea.
There are known in the art granulation apparatus for large diameter granules and these are of the rolling type, compression molding type, casting type, or mechnical processing type. These are however not suitable for mass production.
In the case of apparatus for manufacturing large diameter granules, the quantity of the substance which is necessary to make up the granules is from several times to several hundred times greater than that for making small diameter granules, and generally the granule particle size distribution is broad in such apparatus. Therefore, technologically speaking it is particularly more difficult to obtain products with uniform particle size and shape which have a large diameter than product with a smaller diameter.
Accordingly, in the above-mentioned conventional granulation apparatus, the maximum production per unit apparatus is as small as 500 t/day although the costs for the facilities and the operation are unchangeably high.
There are sometimes demands for products with granule diameter of more than 2 mm for convenience in use, handling or storage, and there are also demands for a method and apparatus which facilitate production of large sized granular products of an arbitrary diameter in large quantities and at less cost in order to meet the need of mixing small sized and large sized granules in a suitable ratio to increase the bagged amount per unit volume and to facilitate handling during use. This will be most convenient for storage, transportation and use of granule products of the same or the different quality.